Unacceptable electrical switching performance can result in switching applications where the actuation force varies slightly below the switch actuating force or slightly above its de-actuating force for indefinite periods of time. For reliable and predictable electrical switching performance, it is desirable to maintain maximum contact force until the point of actuation or de-actuation. In non-snap switches and the vast majority of precision, snap-action switches, contact forces are at a maximum at the plunger free position (i.e., plunger fully extended) and the full over-travel position (i.e., plunger fully depressed).
Contact force diminishes to zero as the switch apparatus approaches the operating point, the plunger position at which the switch changes electrical state from the normally-closed (NC) circuit to the normally-open circuit (NO). Likewise, contact force decreases to zero as the switch apparatus approaches its release point, the plunger position at which the switch changes state from the NO circuit back to the NC circuit. As the contact force varies at or near zero, the switch is susceptible to intermittent non-contact, welding of contacts, and excessive heat generation and contact erosion.
In addition, once actuation or de-actuation commences, it is desirable that the switch apparatus moves from free position to full over-travel position, or vice versa, in one continuous motion. The uninterrupted switch apparatus motion from free position to full over travel results in a minimum amount of time spent near zero contact force and in a maximum relative movement between the moveable contact and the stationary contacts between switching events.
One example of a switching application where the actuating force can vary slightly below the switch actuating force is a mechanical thermostat. A temperature sensitive bimetal spring expands and contracts in response to changes in the surrounding environmental temperature. If a non-snap or typical snap-action switch is directly actuated by the bimetal, it is quite possible the force supplied by the bimetal can vary slightly below the switch actuating force for long periods of time. In such an application, the electrical performance of a non-snap or typical snap-action is likely to be unacceptable.
FIG. 1 illustrates a graph 100 depicting plunger force versus displacement behavior typical of a conventional snap-action switch that may have a total movement range as low as 0.5 millimeters to as high as 2 millimeters for total plunger travel depending on the overall size and design of the snap spring mechanism. The shape of the plunger force-deflection curve increases in linear fashion from a near zero plunger force at the free position (point A) up to operate position (point B) at which time the force between the common moveable contact and the normally closed stationary contact becomes zero. When the switch plunger reaches the operate position (point B), stored energy in the switch apparatus can cause a “snap-over” of the common moveable contact from the normally closed to the normally open stationary contact. The plunger force thereafter drops to point C.
If the plunger is moved further, the plunger force can again increase in linear fashion as indicated by line C-D in FIG. 1. When the plunger is gradually released, the plunger force retraces back along line D-C and continues beyond to the release point E where the contact force between the common moveable contact and the normally open stationary contact again becomes zero. At the release point E, stored energy within the switch apparatus is used to “snap-back” the common moveable contact to the normally closed stationary contact while the plunger force experiences a sudden increase from point E to point F. Releasing the plunger further then causes the plunger force to retrace along line FA back to free position point A.
Low total travel switch designs with nearly linear and positive plunger force deflection spring rates are known as high precision snap-action switches. The linear and increasing plunger force with displacement behavior allows for precise adjustment during production of plunger operate or release force and the amount of differential travel between operate and release positions for the plunger. Differential travel is often adjusted to within 0.0005 inch of a desired value by moving the position of the normally closed stationary contact, which in turn changes the air gap distance the moveable contact, must travel during “snap-over” and “snap-back”. The position of a stationary anchor used to pre-load one member of the snap spring in compression can be moved a small amount to adjust plunger operate or release force to within 10 grams of a desired force level.
Because of precise operating characteristics, low travel, positive rate snap-switch apparatus have often been the switching mechanism of choice when accurate, reliable, and repeatable control of switching functions are required. Such switch control requirements are common in applications involving a pressure or temperature stimulus where accurate and narrow control of a pressure or temperature differential is desired. The low and slow actuation forces produced by pressure and temperature responding resilient members in control applications, however, have caused electrical switching performance issues for conventional high precision, positive rate snap-switch apparatus.
FIG. 2 illustrates a graph 200 depicting plunger force-deflection behavior for many medium travel switch designs having a total plunger travel movement from 2 to 3 millimeters. In graph 200, the plunger force is a substantial value at the switch free position but less than the required plunger force at point B to operate the snap-action mechanism. In the plunger pre-travel range from point A to point B the slope of the curve is still positive but much closer to a zero slope than the low travel, high precision switch designs discussed previously. In the plunger over-travel range from point C to point D the slope of the force-deflection curve may be somewhat positive, zero, or even negative depending on the design of the switch apparatus. Along with the lower slopes, the plunger force-travel curves may exhibit some non-linearity in their shape.
Although medium travel switch apparatus exist with low plunger operate forces, their pre-travel slopes are positive and require a resilient pressure or temperature reacting member to generate an increasing force up to the operate point B in order to actuate the snap switch. A creep-type opening of the electrical contact interface is still a real possibility along with the electrical performance issues.
The low positive slope and nonlinear behavior of the plunger force-deflection curve during plunger pre-travel make it difficult to adjust medium travel switch designs for precise operating characteristics. Many medium travel switch designs are assembled in production without any adjustment of plunger force or differential travel. In addition the differential travel of the moveable contact for a given air gap or “break distance” of a medium travel switch tends to be larger and exhibit more variation than the low travel, high precision switch apparatus. Using a positive rate switch with too large a plunger differential travel for a pressure or temperature control device can unacceptably widen the control range. For these reasons medium plunger travel snap switch designs are not seriously considered for use in pressure and temperature control devices.
FIG. 3 illustrates a graph 300 depicting a force-deflection curve representative of a conventional high plunger travel switch design. The large total plunger travel up to 5 millimeters magnifies the nonlinear shape of the plunger force versus travel curve such that positive and negative slope portions exist in both the pre-travel (point A to B) and the over-travel (point C to D) range for plunger travel. The maximum plunger force during the pre-travel range usually occurs at some distance prior to the plunger travel reaching operate position (point B) or “snap-over”.
The negative slope or decreasing rate of plunger force before reaching operate position is desirable for rapidly moving through the “snap-over” point and into the over-travel region before the plunger again begins to experience increasing force resistance to movement. If total plunger travel movement is restricted to just the negative slope portion of the plunger force versus travel curve then the creep-type opening and closing of the moveable contact will not occur and the unreliable electrical switching performance problems mentioned previously are no longer a concern.
To achieve a negative rate portion for the plunger force-deflection curve centered in the middle of the large total travel range requires rather large and lengthy snap spring geometry. The snap spring length may become 1.5 inches in length or longer and when mounted in some type of case or housing with stationary contacts can grow to 1.8 inches or more in overall length. The length dimension of a high travel switch design usually becomes too large to fit within the space available of many pressure switch and thermostat housing bodies.
FIG. 4 illustrates a graph 400 depicting a contact force, in accordance with an embodiment of the present invention. Graph 400 of FIG. 4 illustrates how contact force, or the force between the moveable contact and stationary contact interface, can vary with the plunger travel position for conventional snap-switch apparatus with low and medium total plunger travel designs. As the switch plunger is actuated from free position (point A) to the operate position (point B) the force of the common moveable contact against the normally closed stationary contact decreases in near linear fashion from free position point A down to zero at the operate position point B.
At the plunger operate position (point B), “snap-over” of the moveable contact to the normally open stationary contact occurs and the contact force is then represented by point C. Depressing the switch plunger further causes contact force on the normally open stationary contact to increase linearly toward the plunger full over-travel position (point D). As the switch plunger is released, the contact force retraces from point D back to point C and beyond to the release position (point E). Here the moveable contact “snaps-back” to the normally closed stationary contact with a force represented by point F. Further release of the switch plunger causes the contact force to retrace from point F to the free position point A.
For high plunger travel switch apparatus, the contact force also diminishes to zero at the switch plunger operate point but may exhibit considerable non-linearity in the shape of the contact force path. The shape of the contact force versus plunger travel curve can become sinusoidal for large travel switch apparatus. The important fact to realize is that as the switch plunger approaches the operating or release position, the contact force decreases and reaches zero at the instant the moveable contact separates from the stationary contact. In switch applications involving slow plunger actuation motion (i.e., creep-like plunger velocity) the switch apparatus can remain near the impending operate position with near zero contact force for a long period of time.
Such a condition can cause non-contact (e.g., dead-break) because not enough force exists at the interface of the mechanically closed contacts to conduct sufficient current to energize the device being controlled by the switch. Low contact force also allows high electrical resistance at the contact interface to develop that can lead to excessive heating and softening and perhaps melting of the contact materials. Once the plunger reaches operate position point B, the moveable contact may not transfer with a sudden snap-like action if plunger velocity is too low.
Because internal pivot or bearing friction may be present within the switch apparatus, the moveable contact may stall for some time during the transfer to the opposite stationary contact side, with unacceptable arcing or a period of electrical non-conduction occurring. After the moveable contact completes the transfer across the air gap and strikes the opposite stationary contact, the moveable contact may bounce off the stationary contact for a short time when plunger actuation velocity is slow. Excessive contact bouncing during contact closure aggravates contact welding, as each successive bounce during closure can generate heat and create an opportunity for a weld to form.
Oftentimes when a positive rate switch apparatus is used to provide the switching function in a slow responding pressure or temperature control device, a snap action interface means is used to help quickly move the switch plunger through the operate and release positions where the contact force goes to zero. Devices are known, for example, where a Belleville spring provides a negative spring rate interface between a pressure driven diaphragm and the switch in order to speed up the switching of the moveable contact and improve electrical performance.
All of the aforementioned designs and configurations provide unacceptable switching performance that can result in switching applications where the actuation force varies slightly below the switch actuating force or slightly above its de-actuating force for indefinite periods of time. Embodiments are thus described herein which overcome such drawbacks.